Since a lithium secondary battery has a high energy density, an application range thereof is not limited to a handheld equipment such as a mobile phone or a personal computer, but is expanded to a hybrid automobile, an electric automobile, an electric power storage system, and the like. As one of these secondary batteries, attention has been recently paid to a lithium-sulfur secondary battery for charging and discharging through a reaction between lithium and sulfur.
There is known, in Patent Document 1, a lithium-sulfur secondary battery which is provided with a positive electrode including a positive electrode active material containing sulfur, a negative electrode including a negative electrode active material containing lithium, and a separator disposed between the positive electrode and the negative electrode.
As a positive electrode of this kind of lithium-sulfur secondary battery, there is known, e.g., in Patent Document 1 a positive electrode which includes a current collector, a plurality of carbon nanotubes which are grown so as to be oriented in a direction perpendicular to the surface of the current collector, and sulfur which covers the surface of each of the carbon nanotubes (in general, the density of a carbon nanotube is 60 mg/cm3, and the weight of sulfur is 0.7 to 3 times the weight of a carbon nanotube). If this positive electrode is applied to a lithium-sulfur secondary battery, an electrolytic solution comes into contact with sulfur over a wide area, with an improved utilization efficiency of sulfur, whereby an excellent charge-discharge rate characteristic and a large specific capacity (discharge capacity per unit weight of sulfur) is obtained.
Here, as a method of covering the surface of each of the carbon nanotubes with sulfur, there is generally known a method in which sulfur is placed at a growing end of the carbon nanotubes, the sulfur is melted, and the melted sulfur is diffused into a base end side through a gap between the respectively adjacent carbon nanotubes. However, in this kind of method, sulfur is present unevenly only near the growing end of the carbon nanotubes, and is not diffused up to the vicinity of the base end of the carbon nanotubes. As a result, there are cases where the vicinity of the base end is not covered with sulfur or may be covered with sulfur having an extremely thin film thickness even when being covered with sulfur. In addition, during discharge, sulfur reacts with lithium to become Li2S, and expands in volume by about 80%. Therefore, the gap between the respectively adjacent carbon nanotubes becomes smaller, and the electrolytic solution is not supplied up to the vicinity of the base end of the carbon nanotubes efficiently. This does not bring about a lithium-sulfur secondary battery having an excellent charge-discharge rate characteristic and a large specific capacity.
Therefore, the inventors of this invention made intensive studies and have come to obtain the following finding. That is, if the density of the carbon nanotubes can be set to a value of 40 mg/cm3 or lower, even in a method that is similar to above, sulfur can be efficiently supplied down to the interface between the current collector and the base end of the carbon nanotubes when sulfur is melted and diffused. In addition, even when sulfur expands in volume during discharge, the electrolytic solution can be supplied down to the vicinity of the base end of the carbon nanotubes efficiently.